The CGEM has a Losmandy D size saddle, so getting a D size plate and bolting the SN6 rings onto it might be the most robust choice.

You can adapt the stock saddle on the CGEM to use the smaller Vixen compatible dovetail bar used on the SN6 ... either by placing an adapter in the stock saddle or by replacing the stock saddle with the wonderful ADM "dual" saddle (which handles either the narrower or wider bars/plates).